Mechanical behavior of brittle-ductile transition in rocks at different depths

The mechanical behavior of brittle-ductile transition in rocks is one of the essential components of deep rock mechanics, as well as a key factor that impacts the safe and efficient implementation of deep rock engineering projects.To study the differences in the mechanical behaviors of brittle-ductile transition in deep rocks, the sandstones, conglomerates and andesites at different occurrence depths (1 000-6 400 m) from the Songliao Basin are taken as research objects, and a number of conventional triaxial tests are carried out on rocks at an identical depth but under different stress levels, rocks at different depths but under an identical stress level, and rocks under different stress levels and at different depths.The brittleness characteristics of rocks are analyzed by using the pre-peak and the post-peak brittleness index.A preliminary understanding of the key influencing factors and differences in the mechanical behaviors of brittle-ductile transition in deep rocks at different depths has been achieved.The research results show that rather than an instantaneous brittle-ductile transition in rocks at different depths, a stress zone exists in progressive transition from brittleness to ductility.The brittleness of rocks at an identical depth but under different stress levels are mainly affected by the confining pressure.For sandstones at a depth of 1 600 m, its brittleness generally decreases with the increase of confining pressure, and there is a transition from its brittleness, ductility to strain hardening, its post-peak plasticity gradually increases until it shows a complete plasticity after the peak.The stress level range of 50 MPa to 70 MPa is the brittle-ductile transition zone of sandstones at the depth of 1 600 m.The brittle ductility characteristics of rocks at different depths under the same confining pressure are mainly affected by their own mineral components.For the rock samples from the Songliao Basin, their content of hard phase minerals and middle phase minerals generally increase with the increase of depths, which causes their brittleness to go up with depth increases, exhibiting a shallow to deep characteristic of transition from brittleness to ductility to brittleness.The post-peak characteristics of in-situ rock stress levels at different depths show different characteristics as the depth increases: the sandstones at depths from 1 000 m to 3 500 m depth show post-peak straining softening, post peak brittleness is found in sandstones at the depth of 4 800 m and conglomerates from depths at 5 100 m to 5 600 m, while the andesites at the depth of 6 400 m manifest the characteristic of post-peak plastic flow.Many factors affect the brittleness and ductility of rocks.Among them, game phenomenon exists in the impact on the brittleness and ductility of rocks between the mineral composition and stress environment.As the depth increases, the increase of hard phase minerals and middle phase minerals will cause the brittleness to increase, and the increase in the loading stress level will inhibit the brittleness of the rocks.The research results are expected to guide the scientific explorations and efficient implementation of the deep rock engineering. © 2021, Editorial Office of Journal of China Coal Society. All right reserved.